Palladium-catalyzed coupling reactions of tetrafluoroethylene with arylzinc compounds

Article abstract of DOI:10.1021/ja109911p

Organofluorine compounds are widely used in all aspects of the chemical industry. Although tetrafluoroethylene (TFE) is an example of an economical bulk organofluorine feedstock, the use of TFE is mostly limited to the production of poly(tetrafluoroethylene) and copolymers with other alkenes. Furthermore, no catalytic transformation of TFE that involves carbon-fluorine bond activation has been reported to date. We herein report the first example of a palladium-catalyzed coupling reaction of TFE with arylzinc reagents in the presence of lithium iodide, giving α,β,β-trifluorostyrene derivatives in excellent yields.

Full text of DOI:10.1021/ja109911p

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Palladium-Catalyzed Coupling Reactions of Tetrafluoroethylene with
Arylzinc Compounds
Masato Ohashi,†,‡ Tadashi Kambara,† Tsubasa Hatanaka,† Hiroki Saijo,† Ryohei Doi,† and Sensuke Ogoshi*,†
†Department of Applied Chemistry, Faculty of Engineering, and ‡Center for Atomic and Molecular Technologies, Osaka University,
Suita, Osaka 565-0871, Japan
S Supporting Information
b
ethylene, giving (η2-CF2dCF2)Pd(PPh3)2 (2) in 96% isolated
yield.13 In contrast, treatment of Pd2(dba)3 with 1 in the
ABSTRACT: Organofluorine compounds are widely used
in all aspects of the chemical industry. Although tetrafluoro-
ethylene (TFE) is an example of an economical bulk
organofluorine feedstock, the use of TFE is mostly limited
to the production of poly(tetrafluoroethylene) and copoly-
mers with other alkenes. Furthermore, no catalytic trans-
formation of TFE that involves carbon-fluorine bond
activation has been reported to date. We herein report the
first example of a palladium-catalyzed coupling reaction of
TFE with arylzinc reagents in the presence of lithium iodide,
giving R,β,β-trifluorostyrene derivatives in excellent yields.
presence of PPh3 (2 equiv relative to Pd) gave an equilibrium
mixture of 2 and (η2-dba)Pd(PPh3)214 even under high-pressure
(3.5 atm) conditions. X-ray diffraction study of 2 clearly demon-
strated that a TFE molecule is coordinated to the Pd atom with a
trigonal-planar coordination geometry (Figure 1).
Addition of Lil to a solution of 2 in THF promoted the
oxidative addition of a C-F bond, which proceeded smoothly to
afford a trifluorovinylpalladium(II) iodide species (3) in nearly
quantitative yield (Scheme 1). In contrast to the known Pt
analogue,11a the C-F bond cleavage on Pd took place without
heating of the reaction mixture. An attempt to cleave the C-F
bond in 2 at 100 °C without LiI resulted in decomposition of
2 along with the liberation of a TFE molecule and the precipita-
tion of Pd black. Thus, cleavage of the C-F bond to generate
trifluorovinylpalladium complex 3 requires LiI. Lithium iodide
would act as a Lewis acid to enhance the elimination ability of
fluorine. Formation of a strong Li-F bond might also be
important for the occurrence of the oxidative addition at room
temperature. The ORTEP drawing of 3 (Figure 2) unambigu-
ously shows that the Pd in 3 adopts a square-planar coordination
geometry and is coordinated to two PPh3 ligands in a trans
manner. Complex 3 is the first example of a mononuclear
trifluorovinyl complex generated by C-F bond cleavage of 1
and having a well-defined structure.15
Trifluorovinylpalladium complex 3 seemed to be a promising
reaction intermediate for the preparation of various trifluorovinyl
derivatives via cross-coupling reactions of TFE with organome-
tallic reagents. In particular, each reaction step in the cross-
coupling of TFE with arylmetal reagents to give trifluorovinylar-
enes must occur at a relatively low temperature, since undesired
side reactions of the resulting trifluorovinylarenes give a complex
mixture. In fact, [2 þ 2] cyclodimerization of R,β,β-trifluoros-
tyrene occurs in a head-to-head manner at 80 °C to give a mixture
of cis and trans isomers.16 Therefore, 3 was reacted with a
stoichiometric amount of ZnPh2 (4a) to determine whether
the expected reaction to give R,β,β-trifluorostyrene (5a) would
occur at room temperature. Although the reaction of 3 with 0.5
equiv of 4a in THF resulted in a complicated mixture that
contained a small amount of the expected compound 5a, the
reaction of 3 with 4a in the presence of Lil and trans,trans-
dibenzylideneacetone (DBA) proceeded smoothly to give 5a in
87% yield (Scheme 1). Both Lil and DBA are potential additives
rganofluorine compounds are important components of a
O
variety of commercial products.1 Therefore, an economical,
bulk organofluorine feedstock is needed as the starting material
for the synthesis of commercial products. Tetrafluoroethylene
(TFE) is an ideal starting material because it is industrially an
economical bulk organofluorine feedstock for the production of
poly(tetrafluoroethylene) and copolymers with other alkenes.2
However, only a few reactions that substitute carbon nucleo-
philes for a fluorine on TFE to form a new C-C bond have been
reported.3 Moreover, the previously described reactions suffer
from undesired side reactions, even at low reaction tempera-
tures.3a,3b Recently, homogeneous catalytic reactions involving
C-F bond activation have received increasing attention.4-9
However, to the best of our knowledge, no catalytic reactions
that involve TFE have been reported.10 Therefore, the develop-
ment of a selective transition-metal-catalyzed transformation of
TFE via C-F bond activation would be a remarkable break-
through that would greatly increase the potential of TFE as a
useful starting material for a variety of organofluorine com-
pounds. To date, C-F bond activation in TFE has been achieved
only in a few stoichiometric reactions.11 In a groundbreaking
study of C-F bond activation in TFE, Hacker, Littlecott, and
Kemmitt11a reported that LiI promotes the oxidative addition of
TFE to Pt(0). This observation inspired us to develop a transi-
tion-metal-catalyzed cross-coupling reaction of TFE with orga-
nometallic compounds. Herein we report the formation,
structure, and reactivity of a trifluorovinylpalladium complex
obtained by oxidative addition of the C-F bond of TFE to Pd(0)
promoted by LiI. Moreover, a novel Pd-catalyzed coupling
reaction of TFE with arylzinc compounds is also discussed.
Reaction of (η2-CH2dCH2)Pd(PPh3)2 with TFE (1; 1
12
Received: November 4, 2010
Published: February 15, 2011
atm) in toluene for 1 h resulted in substitution of the coordinated
r
2011 American Chemical Society
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dx.doi.org/10.1021/ja109911p J. Am. Chem. Soc. 2011, 133, 3256–3259
|

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Scheme 2. Plausible Reaction Mechanism
Figure 1. Molecular structure of 2 with thermal ellipsoids at the 30%
probability level. H atoms and solvated molecules (toluene) have been
omitted for clarity.
Although either elongation of the reaction time or elevation of
the reaction temperature was required, reduced catalyst loading
increased the yield of 5a (entries 3 and 4). Even in the presence of
0.01 mol % Pd2(dba)3, the catalytic reaction proceeded smoothly
at 40 °C to give 5a in 72% yield (entry 4). The rate of the
coupling reaction was remarkably enhanced by omission of PPh3
from the catalytic system, and 5a was obtained in 73% yield
(entry 5). Under the same reaction conditions, the reaction with
isolated ZnPh2 occurred somewhat slowly relative to that with
ZnPh2 prepared in situ (entry 6). In contrast, in the absence of
Pd(0), the reaction of 1 with ZnPh2 was negligible. This result
clearly indicates that Pd(0) catalyzes the coupling reaction with
or without Lil (entries 7 and 8).
With the optimized reaction conditions, the scope of the
diarylzinc reagent was investigated. Treatment of 1 with 4a
prepared by the reaction of ZnCl2 with PhMgCl gave 5a in
81% yield, which was the highest reactivity among the arylzinc
reagents (turnover number = 8100; entry 9). It should be
mentioned that in comparison with PhMgBr (entry 5), the use
of PhMgCl for the preparation of 4a resulted in a higher yield of
5a. In addition, the reactions with Zn(4-MeC6H4)2 (4b) and
Zn(3-MeC6H4)2 (4c) gave the monoaryl-substituted products
5b and 5c in yields of 75 and 72%, respectively (entries 10 and
11), while the reaction with Zn(2-MeC6H4)2 (4d) gave only a
57% yield of 5d (entry 12). Reactions with fluoro-substituted
arylzinc reagents (4e and 4f) yielded the corresponding products
(5e and 5f) in 53 and 55% yield, respectively (entries 13 and 14).
Reactions with para-substituted arylzinc reagents such as Zn(4-
MeOC6H4)2 (4g) and Zn(4-styryl)2 (4h) also afforded the
corresponding products (5g and 5h) in good yields (entries 15
and 16). In contrast, Zn(4-CF3C6H4)2 (4i), Zn(4-MeSC6H4)2
(4j), and Zn(4-ClC6H4)2 (4k) required prolonged reaction
times to yield the corresponding monoaryl-substituted products
(5i-k) in moderate yields (entries 17-19). In addition, the
reaction with Zn(4-Me2NC6H4)2 (4l) was terminated within 2 h,
and consequently, the yield of the desired product (5l) remained
at 30% (entry 20). This catalytic system was also successfully
applied to Zn(2-naphthyl)2 (4m), which gave the corresponding
product (5m) in 61% yield (entry 21). Use of Zn(2-thienyl)2
(4n) allowed the reaction with 1 to proceed to give 5n, although
a much longer reaction time was required and the product yield
was low (entry 22). Reaction products were isolated in THF
solution to avoid the occurrence of cyclodimerization to give
Figure 2. Molecular structure of 3 with thermal ellipsoids at the 30%
probability level. H atoms have been omitted for clarity.
Scheme 1. Transformation of TFE to R,β,β-Trifluorostyrene
via C-F Bond Cleavage on Palladium
in the cross-coupling reaction, since the reaction of TFE with
Pd2(dba)3 and PPh3 in the presence of LiI to give 3 simulta-
neously yields uncoordinated DBA. The role of Lil in this
reaction is the formation of reactive zincates such as Li[ArZnXI]
(X = Ar, I; see below).17 In contrast, Pt is unlikely as a catalyst for
the cross-coupling reaction because the oxidative addition of
TFE to Pt(0) requires both a much higher temperature and a
longer reaction time (95 °C for 24 h).11a
It was a logical extension of this reaction scheme to conduct a
Pd-catalyzed coupling reaction of 1 with 4a in the presence of Lil,
and the results are summarized in Table 1. In the presence of 5
mol % Pd2(dba)3 and 20 mol % PPh3, the coupling reaction of 1
with 4a (which was prepared in situ by treating ZnCl2 with 2
equiv of PhMgBr) took place at room temperature. The desired
product 5a was obtained in 55% yield (entry 1). As expected
from the stoichiometric reactions, the addition of Lil was
essential for the Pd-catalyzed coupling reaction (entry 2).
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COMMUNICATION
Table 1. Pd(0)-Catalyzed Coupling Reaction of TFE (1) with ZnAr2 (4)a
ZnAr2
product
entry
Pd2(dba)3 (mol %)
PPh3 (mol %)
LiI (mmol)
4
Ar
time (h)
5
yield (%)b
1c
2c
3c
5.0
5.0
20.0
20.0
4.0
0.04
-
0.240
-
4a
4a
4a
4a
4a
4a
4a
4a
4a
4b
4c
4d
4e
4f
Ph
20.0
50.0
46.0
24.0
4.0
5a
5a
5a
5a
5a
5a
5a
5a
5a
5b
5c
5d
5e
5f
55
Ph
22
1.0
0.240
0.240
0.240
0.240
0.240
-
Ph
58
4
0.01
0.01
0.01
-
Ph
72
5
6d
Ph
73 (52)
60
-
Ph
9.0
7
-
Ph
24.0
24.0
2.0
3
8
9e
-
-
Ph
9
0.01
0.01
0.01
0.01
0.01
0.01
0.01
0.01
0.01
0.01
0.01
0.01
0.01
0.01
-
0.240
0.240
0.240
0.240
0.240
0.240
0.240
0.240
0.240
0.240
0.240
0.240
0.240
0.240
Ph
81 (62)
75 (34)
72 (45)
57 (30)
53 (45)
55 (51)
63 (39)
65 (71)
31 (14)
41 (37)
37 (10)
30 (25)
61 (37)
34 (14)
10
11e
12e
13
14
15
16
17
18
19
20
21
22
-
4-Tol
3-Tol
2-Tol
6.0
-
4.0
-
8.0
-
3-FC6H4
8.0
-
4-FC6H4
4.0
-
4g
4h
4i
4-anisyl
2.5
5g
5h
5i
-
4-styryl
4.0
-
4-CF3C6H4
4-MeSC6H4
4-ClC6H4
4-Me2NC6H4
2-naphthyl
2-thienyl
18.0
21.0
28.0
2.0
-
4j
5j
-
4k
4l
5k
5l
-
-
4m
4n
4.0
5m
5n
-
75.0
a General conditions: 1 (3.5 atm, >0.30 mmol as estimated from an equation of state), 4 (0.100 mmol, prepared in situ by treating ZnCl2 with 2 equiv of
ArMgBr) and solvent (0.5 mL). All of the reactions were conducted in a pressure-tight NMR tube. b Yields are based on the aryl group and were
determined by 19F NMR analysis of the crude product using R,R,R-trifluorotoluene as an internal standard. The values in parentheses are isolated yields.
Detailed procedures for the isolation experiments are provided in the Supporting Information. c Run at room temperature. d Run with isolated 4a (0.100
mmol, purchased from Strem) e Run with ArMgCl instead of ArMgBr (0.200 mmol).
hexafluorocyclobutane derivatives at higher concentrations.18 The
relatively lower isolated yields were caused either by the high volatility
even at room temperature or by the cyclodimerization.
achieved by the synergetic effects of the Pd(0) species and LiI
and generated a trifluorovinyl palladium(II) intermediate whose
structure was unambiguously determined by X-ray diffraction.
Further studies of the application of the C-F bond activation of
TFE to other catalytic transformations are in progress in our group.
On the basis of the results described above, the Pd-catalyzed
monoaryl substitution reaction of TFE might proceed via the
mechanism depicted in Scheme 2. Coordination of a TFE
molecule to Pd(0) takes place, generating an η2-TFE species
(B). Next, oxidative addition of a C-F bond to Pd(0) is
promoted by Lil, generating a trifluorovinylpalladium(II) inter-
mediate (C). Transmetalation of C with Li[ArZnXI] yields a
transient arylpalladium intermediate (D), which undergoes re-
ductive elimination to afford R,β,β-trifluorostyrene derivative 5
along with regeneration of the Pd(0) species. Addition of Lil is
essential not only for accelerating the cleavage of the C-F bond
but also for enhancing the reactivity of the arylzinc reagent via the
formation of a zincate such as Li[ArZnXI].
’ ASSOCIATED CONTENT
S
Supporting Information. Experimental details and char-
b
acterization data. This material is available free of charge via the
Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION
Corresponding Author
ogoshi@chem.eng.osaka-u.ac.jp
In summary, we have demonstrated the first Pd-catalyzed
monoarylation of TFE by employing in situ-prepared diarylzinc
reagents to yield R,β,β-trifluorostyrene derivatives in excellent
yields with high selectivity. C-F bond activation of TFE was
’ ACKNOWLEDGMENT
This work was supported by a Grants-in-Aid for Scientific
Research (21245028) and Encouragement for Young Scientists
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COMMUNICATION
(B) (21750102) from MEXT. We express our thanks to Daikin
Industries, Ltd., for the supply of tetrafluoroethylene. S.O. and
M.O. acknowledge the NAGASE Science and Technology
Foundation and the Japan Petroleum Institute, respectively, for
the Grants for Research.
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7923–7925. (b) Tellier, F.; Sauvetre, R.; Normant, J. P.; Dromzee, Y.;
Jeamnin, Y. J. Organomet. Chem. 1987, 331, 281–298.
(17) Achonduh, G. T.; Hadei, N.; Valente, C.; Avola, S.; O’Brien,
C. J.; Organ, M. G. Chem. Commun. 2010, 46, 4109–4111.
(18) Heinze and Burton claimed that some R,β,β-trifluorostyrene
derivatives (especially in pure form) are prone to cyclodimerize under
higher-concentration conditions even at room temperature or below.
See: Heinze, P. L.; Burton, D. J. J. Org. Chem. 1988, 53, 2714–2720.
’ NOTE ADDED AFTER ASAP PUBLICATION
The new bond identified in the fourth sentence of the first
paragraph was corrected February 18, 2011.
(6) For transition-metal-catalyzed reactions via C-F bond activa-
tion of alkyl fluorides, see: (a) Kiplinger, J. L.; Richmond, T. G. J. Am.
Chem. Soc. 1996, 118, 1805–1806. (b) Burdeniuc, J.; Crabtree,
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